In recent years, there are many situations that require sensor systems such as a need for, due to the spread of networks, personal authentication to avoid security risks, automatic operation of vehicles, and the spread of a so-called “internet of things” (IOT). There are many kinds of sensors and a variety of information detected thereby. One of them is a method for obtaining information by irradiating a target with light from a light source and receiving information from reflected light. For example, a pattern recognition sensor and an infrared radar are provided.
As the light source of the sensors, light having features tailored to the intended application, which are wavelength distribution, luminance and profile, is used. As the wavelength of the light, visible light to infrared light are used often. Especially, infrared light is widely used since it is characterized by being resistant to the influence of outside light, being invisible, and making it possible to observe the slight inside of a target. For the type of the light source, a LED light source and a laser light source are used often. For example, to detect a distant place, a laser light source is preferably used, which is small in light spread. To detect a relatively near place or irradiate a relatively wide region, an LED light source is preferably used.
The size and shape of a target irradiation region is not always consistent with the spread (profile) of light from a light source. In this case, it is necessary to regulate the shape of the light by a diffuser plate, lens, shield plate, etc. Recently, a so-called light shaping diffuser (LSD) was developed, which is a diffuser plate that can shape the shape of light to some extent.
As another light shaping method, a diffractive optical element (DOE) is provided, which utilizes a diffraction phenomenon that is observed when light passes through an area where materials with different refractive indices are periodically arranged. The DOE is basically designed with respect to a single wavelength of light and, in theory, it can shape light in an almost desired shape. With the above-mentioned LSD, the intensity of light in an irradiation region has a Gaussian distribution. Meanwhile, the DOE can control the uniformity of light distribution in an irradiation region. Such a characteristic of the DOE is advantageous in increasing efficiency by reducing light irradiation to an unnecessary region, and in downsizing a device by, for example, decreasing the number of light sources (for example, see Patent Document 1, etc.)
The DOE is applicable to both a directional light source such as laser and a diffused light source such as LED. The DOE is also applicable to a wide range of wavelengths including ultraviolet light, visible light and infrared light.
The DOE needs nanometer-scale microfabrication. Especially, to diffract a long wavelength of light, it is needed to form a high-aspect-ratio fine shape. Therefore, an electron beam lithography technique using an electron beam, has been used for the production of the DOE. For example, a desired DOE can be obtained as follows: a hard mask and a resist film are formed on a quartz plate, which is transparent in ultraviolet to infrared ranges; a predetermined shape is photolithographically created in the resist by using an electron beam, followed by developing the resist, dry-etching the hard mask, and then dry-etching the quartz, thereby forming a pattern on a surface of the quartz plate; then, the hard mask is removed, thereby obtaining the desired DOE.